The main problem under consideration is to determine what controls the development and modification of connections between units in excitable systems. We will use three primary preparations that have small numbers of units and whose nerve and muscle cells have been specifically identified. (1) A major preparation will be the giant reticulo-spinal neurons (Muller cells) of the lamprey brain. We have recently identified and characterized the dendritic tree for several of these cells. We will study the factors that control the development and form of the dendritic tree in these identified cells by working with embryonic material. (2) We will examine trophic interactions between excitable units by looking at the relationship between nerve and muscle in regenerating limbs of the cockroach. We will also examine the development and breakdown of neuromuscular connections in the moth by following the fate of identified nerve and muscle cells through critical stages of development. (3) The relationship between cell metabolism and electrical properties of the cell membrane will be examined in central neurons of insects using freeze-fracture electron microscopy. The soma and the axon of identified insect central neurons have different electrical properties. We can change the electrical properties either by injury to the axon or by treatment with colchicine. We will study the ultrastructural changes in the membrane that may be correlated with these differences in electrical responsiveness.